The present invention relates to a liquid crystal display device (LCD) having excellent display quality and a method for manufacturing the same. More particularly, the present invention relates to a vertical alignment mode LCD having high display quality and a convenient, useful method for manufacturing the same.
Currently, the LCDs are used as display devices for various information processors including personal computers and navigation systems. The current mainstream is TN (Twisted Nematic) mode LCDs, one of the horizontal alignment modes.
In the liquid crystal layer of the TN mode LCDs, liquid crystal molecules having positive dielectric anisotropy are provided between a pair of substrates so as to be aligned approximately horizontally with respect to the substrate surface. The alignment direction of the liquid crystal molecules at one of the substrates is twisted by 90.degree. from that of the liquid crystal molecules at the other substrate. The TN mode LCDs provide display in so-called normally white mode. More specifically, the TN mode LCDs provide white display in a turned-off state, and black display in a turned-on state. However, satisfactory black display is less likely to be obtained with the TN mode LCDS. The reason for this is as follows: in the TN mode LCDs, even in a turned-on state (i.e., when a voltage is applied), the liquid crystal molecules located close to the substrate are not oriented in parallel with an electric field (vertically to the substrate) and maintain their horizontal alignment. Thus, light with its polarization plane rotated by the birefringence of such horizontally aligned liquid crystal molecules passes through the liquid crystal panel.
In recent years, practical applications of vertical alignment mode LCDs have been studied. The vertical alignment mode LCDs are the LCDs in which liquid crystal molecules having negative dielectric anisotropy are aligned vertically to the substrate surface. The vertical alignment mode LCDs are advantageous over the TN mode LCDs in that high-quality black display is more easily obtained and thus high contrast ratio display is more easily realized. In the vertical alignment mode, almost all liquid crystal molecules are aligned approximately vertically to the substrate surface in a turned-off state. Therefore, the liquid crystal layer does not rotate the polarization plane of light, whereby nearly perfect black display can be realized.
However, the vertical alignment mode LCDs have the following problem.
In the vertical alignment mode, the liquid crystal molecules are aligned approximately vertically to the alignment film surface. Therefore, it is difficult to apply the alignment regulation force in an azimuth direction to the liquid crystal molecules. As a result, high-quality white display is less likely to be obtained when the liquid crystal molecules are oriented horizontally (vertically to an electric field) by applying a voltage to the liquid crystal layer. This problem will now be described with reference to FIGS. 11 and 12.
FIG. 11 is a plan view of a conventional vertical alignment LCD 400. FIG. 12 is a cross-sectional view of the LCD 400 in a turned-on state (i.e., when a voltage is applied). FIG. 12 corresponds to a cross-sectional view taken along line 12A-12A' of FIG. 11.
The LCD 400 has a liquid crystal layer 30 between a TFT (Thin Film Transistor) substrate 10 and a color filter substrate 20. The liquid crystal layer 30 has liquid crystal molecules 32 having negative dielectric anisotropy. The TFT substrate 10 has a glass substrate 11, gate lines 12 formed thereon, source lines 14, and picture-element electrodes 18 each connected to a corresponding source line 14 through a corresponding TFT 16. The color filter substrate 20 has a glass substrate 21, a color filter layer 27 formed thereon, and a counter electrode 28. Each of the TFT substrate 10 and the color filter substrate 20 has a vertical alignment film 42 at its surface facing the liquid crystal layer 30. Note that the hatched regions of the color filter layer 27 represent a black matrix.
When the LCD 400 is in the driven state, an electric field (arrow E) is generated between the picture-element electrode 18 and the gate line 12, as shown in FIG. 12. As a result, the liquid crystal molecules 32 located near the gate lines 12 are tilted in the directions shown by arrows A and B according to the intensity of the electric fields. Similarly, in the cross section taken along line 12B-12B' of FIG. 11, the liquid crystal molecules 32 located near the source lines 14 are tilted in response to the electric field generated between the source line 14 and the picture-element electrode 18.
Thus, the liquid crystal molecules 32 located near the gate lines 12 and source lines 14 tend to be tilted in different directions, respectively. In other words, if the liquid crystal molecules 32 are not subjected to alignment regulation in the azimuth direction (the direction defined within the display plane or within the plane of the liquid crystal layer), the liquid crystal molecules 32 located in the corresponding portions tend to be tilted in response to the electric fields, so that four regions are produced where the liquid crystal molecules 32 are oriented in different azimuth directions, respectively. However, these four regions are unstable and also the effect of the electric field in each picture-element region is not necessarily uniform. Therefore, these four regions cannot be formed stably at a desired area ratio in each picture-element region. Such variation in the area ratio of the four regions degrades the display quality. This is visually recognized as unevenness of the display when the LCD is viewed obliquely.
An example of the most common, convenient method for controlling the alignment direction of the liquid crystal molecules (hereinafter, the alignment direction refers to the azimuth alignment direction unless otherwise specified) is a rubbing method in which the alignment film is rubbed to provide a slight tilt angle in the vertical alignment (the tilt angle as used herein is an angle from the normal of the alignment film surface, and indicates displacement from the vertical alignment). In order to obtain the alignment regulation force exceeding the effect of the electric field, a tilt angle of about 3.degree. or more is generally required. However, the vertical alignment film subjected to the rubbing method does not have enough alignment regulation force. Therefore, a stable tilt angle is less likely to be obtained by the rubbing method. In other words, a slight difference in rubbing conditions results in variation in tilt angle of the liquid crystal molecules within the display plane, and this variation in tilt angle is visually recognized as stripe-shaped alignment defects.
Other methods for obtaining the alignment regulation force include a method utilizing an electric field (Japanese Laid-Open Publication Nos. 6-301036 and 7-230097), a method utilizing the effect of uneven shape of the surface facing the liquid crystal layer (IDW '97, p. 159, "A Vertically Aligned LCD Proving Super-High Image Quality"), and the like. However, none of these methods are desirable due to insufficient alignment regulation force, an increased number of manufacturing processes, or the like.
Moreover, Japanese Laid-Open Publication No. 7-64092 discloses a method for preventing generation of a disclination line at the boundary between two or more differently aligned regions within a single picture-element region of the normally white mode TN display. More specifically, a vertically aligned region is formed at the boundary between the differently aligned regions in order to prevent generation of the disclination line. In this method, however, alignment of the picture-element region is partially changed at the vertically aligned region, and the alignment film in the vertically aligned region does not have force to regulate the alignment direction of the liquid crystal molecules and thus does not stabilize the alignment of the liquid crystal molecules in a single direction.